Ribbon cartridge for an electronic typewriter
TECHNICAL BACKGROUND 
     The present invention concerns a ribbon cartridge for an electronic typewriter, and more particularly means for winding the ribbon with suitable tensioning and without the ribbon being folded. 
     In an electronic typewriter, if the hammer mounted on a carrier movable along a platen strokes a spoke of a print wheel, the character formed on the spoke collides along with an inked ribbon with a paper on the platen, thereby printing the character thereon. The ribbon is received into a cartridge, and mounted on the cartridge holder of the carrier. As the carrier moves along the platen, a ratchet mechanism causes the ribbon to be wound in a given direction. 
     Conventionally, a ribbon cartridge includes a supplying spool for supplying the ribbon and a winding spool for winding the ribbon supplied from the supplying spool. The drawbacks of the conventional ribbon cartridge are as follows: 
     First, the ribbon wound around the winding spool is frequently folded so that the contact of the winding spool and winding gear is adversely affected, thereby resulting in improper supplying of the ribbon; 
     Second, the construction of the resilient pressing means for forcefully contacting the moving winding spool, as the volume of the ribbon wound increases, with a winding gear is complicated; 
     Third, it is impossible to adjust the resilient force of the tension spring for tensioning the ribbon released from the supplying spool in accordance with the material and the width of the ribbon; and 
     Fourth, the ribbon is plastically strained and becomes loosely elongated by the hammer stroking. If the elongated ribbon is loosely positioned in front of the hammer, the stroked portion of the ribbon can not pass the position of the hammer and therefore, the next character printed becomes dimmer. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved ribbon cartridge for winding the ribbon with suitable tensioning and without the ribbon being folded. 
     It is a further object of the present invention to provide a ribbon cartridge capable of adjusting resilient force of the tension spring mounted on a supplying spool and simplifying the construction of the resilient pressing means for forcefully contacting the winding spool with the winding gear. 
     It is an another object of the present invention to provide a ribbon cartridge capable of minimizing the deformation of the ribbon. 
     According to the present invention, a ribbon cartridge for an electronic typewriter comprises a case for containing the ribbon, the case having an outlet opening for letting out the ribbon and an inlet opening for letting in said ribbon, the two openings being spaced apart from each other enough to receive therebetween the hammer for pressing the ribbon to print characters, the two openings each having a guide for guiding the ribbon, the guide being sloped symmetrically to each other relative to the vertical plane intersecting the case along the meridian line between the two openings, a supplying spool for supplying the ribbon positioned behind the outlet opening in the case, a winding spool for winding the ribbon supplied from the supplying spool, the winding spool being positioned behind the inlet opening in the case, a winding gear for rotating the winding spool by resiliently contacting the winding spool, a tension member comprising a substantially L-shaped spring, the middle curved portion of the L-shaped spring being supported on the spindle of the supplying spool, a first branch of the L-shaped spring being fixed on a corner structure formed on the corner of the case, a second branch of the L-shaped spring holding the ribbon released from the supplying spool to tension the ribbon, the corner structure comprising a plurality of tension adjustment slots so that the second branch can hold the ribbon with variable tension force according to the position of the slot connected with the first branch, a resilient pressing means for contacting the winding spool with the winding gear, the resilient pressing means formed by a pin spring resiliently pressing the spindle of the winding spool toward the winding gear, a long guide groove formed for guiding the spindle of the winding spool as the ribbon wound around the winding spool is expanded so as to increase the space between the winding spool and the winding gear, and a guide roll disposed in front of the winding spool to guide the ribbon supplied from the supplying spool, a side of the guide roll being tapered bidirectionally from the center line between both ends thereof toward the both ends so that the ribbon is guided without being folded longitudinally. 
     The present invention will now be described more specifically with reference to the drawings attached only by way of example. 
    
    
     BRIFE DESCRIPTION OF THE ATTACHED DRAWINGS 
     FIG. 1 is a perspective for illustrating the inventive structure, the upper plate being removed for showing the inside thereof; 
     FIG. 2 is a plane view of FIG. 1 for illustrating the ribbon winding and the tensioning adjustment; 
     FIG. 3 is a frontal elevation of FIG. 1 for illustrating the ribbon stretching; and 
     FIG. 4 is an operational view for illustrating the ribbon in contact with the guide roll. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Inside case 2 are rotatably mounted winding spool 4 and supplying spool 6, and the ribbon 8 is wound around the supplying spool 6 and moves to and around the winding spool 4. On the corner behind the winding spool 4 inside the case 2 is fixed a supporting protuberance 10 on which pin spring 12 is supported. One end 12a of the pin spring 12 presses resiliently the spindle 14 of the winding spool 4, while the other end 12b of the spring 12 is supported on the case 2. 
     To one side of the winding spool 4 is disposed the winding gear 16. By the resilient force of the spring 12 does the winding spool 4 contact continuously with the winding gear 16. Guide roll 20 is rotatably mounted on guide shaft 22 fixed adjacent to inlet opening 18 inside the case 2. The side of the guide roll 20 is tapered bidirectionally from the center line between the both ends of the guide roll toward the both ends. Namely, the portion along the center line between the both ends of the guide roll 20 is bulged, thereby forming a large diameter. 
     Tension member comprises a substantially L-shaped spring 26. The middle curved portion of the L-shaped spring 26 is supported on the spindle of the supplying spool 6. A first branch 26a of the spring 26 is fixed on a corner structure formed at the corner adjacent to the supplying spool 6. The corner structure comprises a plurality of tension adjustment slots 28. The branch 26a is inserted into one of the slots 28. A second branch 26b of the spring 26 holds the ribbon released from the supplying spool to tension the ribbon. 
     The ribbon 8 released from the supplying spool 6 is wound around the winding spool 4 by passing under the other end 26b of the spring 26, a guide piece 30 and outlet opening 32. The inlet opening 18 of the winding spool 4 and the outlet opening 32 of the supplying spool 6 have respectively a guide 34a, and 34b which is sloped symmetrically to each other relative to the vertical plane intersecting the case 2 along the meridian line between the two openings 18, 32. The reference numeral 36 indicates a long guide groove formed in the case 2 for guiding the spindle of the winding spool 4. 
     Referring to FIG. 2, as the winding gear 16 rotates in one direction by the ratchet mechanism (not shown) of the carrier, the ribbon released from the supplying spool 6 is wound around the winding spool 4. Here, the resilient force of the tension spring 26 for tensioning the ribbon 8 between the two guides 34a, 34b can be adjusted by selecting one of the slots 28 connected with the branch 26a. Therefore, the resilient force of the spring 26 can be suitably adjusted according to the width and material of the ribbon 8. Of course, when the tension spring 26 is itself deteriorated, it is also possible to improve the resilient force thereof by selecting the slot connected with branch 26a. 
     The part of the ribbon 8 stretched between the two guides 34a, 34b is plastically deformed by the hammer stroking, and becomes looser toward the platen (the direction of the arrow shown in FIG. 2). However, since the guides 34a, 34b are sloped symmetrically to each other so as to make the upper edge narrower than the lower edge of the part of the ribbon between the two guides as shown in FIG. 3, the resilient force of the tension spring 26 is concentrated on the lower edge 8b more than on the upper edge 8a of the ribbon 8, thereby the deformation occuring mainly in the lower edge 8b of the ribbon 8. Consequently, when the ribbon 8 is plastically strained by the hammer stroking, the lower edge 8b of the ribbon 8 becomes loosened by plastic deformation, while the upper edge 8a does not lose its inherent elasticity, recovering its original shape so as to make the used ribbon portion pass the position of the hammer. Thus, it is prevented that the printing of a character is dimmed owing to the hammer stroking the used ribbon portion. 
     Of course, if the guides 34a, 34b are sloped symmetrically to each other so as to make the lower edge narrower than the upper edge of the part of the ribbon between the two guides, the similar effect is produced. The sloping angle of the guides 34a, 34b is preferably within the range of 3-7 degrees. 
     The ribbon 8 is wound around the winding spool 4 by the winding gear 16, guided without being folded by the guide roll 20. Namely, as shown in FIG. 4, since the bulged portion 24 of the guide roll 20 causes the ribbon 8 to be bent longitudinally along the center line of the ribbon 8, the ribbon can not be folded, and the folded portion, if any, is spread out, guided to the winding spool 4. 
     Additionally, since the winding spool 4 is resiliently contacted with the winding gear 16 by the end 12a of the spring 12, its construction is simplified. 
     While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that modifications in detail may be made without departing from the spirit and scope of the invention.